1. Field of the Invention
This invention relates to methods and apparatus for the manufacture of flexible polyurethane foams by the essentially free rise, continuous casting method in which carbon dioxide from sources other than the reaction of water and free isocyanate is present in the gaseous state during the foam-forming reaction and contributes to the rise of the foam and lowering of the density of the foam product.
2. Discussion of the Prior Art
Polyurethane foams in general, and flexible foams specifically, use the reaction of isocyanates containing two or more isocyanate (NCO) groups with water to generate carbon dioxide gas. This gas is dispersed and retained to a large extent during the polymerization reaction and thus leads to the formation of foams. The polymerization reaction involves the reaction of additional isocyanate with a polymer that contains NCO reactive groups which are usually OH, but can also include NH.sub.2, SH, COOH and others.
At the time the reaction is essentially complete the foam gels, i.e., becomes solid, and the cells are usually broken open by the heat of reaction and pressure of the trapped gases to produce the well known open cell structure of such flexible foams.
The many considerations relating to the selection of raw materials, catalysts, emulsifiers additives and mechanical conditions to produce a large variety of flexible foams exhibiting different physical properties are well established and known in the art.
The most critical of these properties is the control of density and firmness of foams for various end uses.
The density of the finished foam is controlled to a large extent by controlling the amount of gas generation, or "blowing" by CO.sub.2 by adjusting the amount of water used in the formulation. However, it is well known that the reaction which generates CO.sub.2 also produces urea linkages in the polymer which have the effect of making the foam firmer and more brittle. Therefore the density cannot be varied independently of the effect on other properties simply by addition of more water and isocyanate. Furthermore the reaction of water with isocyanates is highly exothermic and the heat generated can cause undesirable effects ranging from internal degradation, e.g., scorch, to decomposition and even combustion of the foam during the curing phase of the reaction.
To counteract some of these negative aspects so called "auxiliary blowing agents" have been used to produce foams of relatively low density. These are low boiling liquids which are converted to gases by the heat of the polymerization reactions. Since these gases are not incorporated into the polymer structure they do not cause firming or embrittlement. Furthermore, both the boiling of the liquid and subsequent expansion of the gas provide a heat sink to reduce the possibility of thermal degradation of the foam. Thus, if no auxiliary blowing agent is present in the reaction mixture, there is an upper limit on the amount of water that can safely be used in foaming reaction without risking scorching or more extreme thermal degradation.
The choices of liquids to serve this purpose, aside from economic considerations, are limited. The product must boil within a narrowly defined temperature range, be a nonsolvent for the foam, and have no catalytic activity or deleterious effect on the reaction. For obvious reasons, it is preferably nontoxic and nonflammable to avoid hazards in handling and due to emissions both within and outside the plant area. The auxiliary blowing agent should also not leave any residual odor in the foam.
Up to now the commercial blowing agents of choice have been low boiling chlorofluorocarbons (CFC's) such as dichlorodifluoromethane (R-12), trichlorofluoromethane (R-11) and similar products, as well as methylene chloride. The latter and R-11 have been the preferred auxiliary blowing agent for use in flexible foams.
Recently, the continued use of these auxiliary blowing agents has been questioned because of safety and environmental concerns. Methylene chloride is considered potentially toxic and possibly a low potency animal carcinogen, and exposure and allowed concentration in air is likely to be restricted severely thus making it impractical for the industry to continue its use. R-11 and similar CFC's are believed to be involved in potential destruction of ozone in the upper stratosphere with possible long term deleterious health and climatic effects.
There have been many attempts with other materials to achieve the same effects without use of the common auxiliary blowing agents. Because of the narrow range of properties for acceptable low boiling liquid blowing agents, none have been adopted commercially.
Various other methods have also been suggested in the art to eliminate or reduce the need for these commercial auxiliary blowing agents. One such alternative proposes the use of carbon monoxide produced by the decomposition of formic acid as an auxiliary blowing agent. This method has not found acceptance because formic acid is a highly corrosive product and the gaseous degradation product, carbon monoxide, is both highly toxic and flammable with a very wide range of explosive limit (12.5-74.2%) in air.
Another known method for lowering foam density involves pouring foam into molds or like containers which are placed in a sealed chamber. Reduction of atmospheric pressure within the chamber during the rise of the foam produces a finished product of reduced density without addition of gas producing constituents. Such treatment is difficult to control and is not applicable to the majority of commercial manufacturing facilities which produce continuous buns or blocks of flexible polyurethane foam.
Technology exists for producing stable emulsions of gases, including air, in rising foam. This frothing method requires use of pressurized foaming equipment and is not suitable for the preparation of low density and/or soft foams.
It has also been found that the presence of air and other dissolved gases in the foam-forming ingredients, before or during mixing results in the formation of bubbles and unacceptable holes in the cured foam product.
Other alternatives to the use of water and excess polyisocyanate, and CFC and other hydrocarbon blowing agents, which have been suggested include carbon dioxide among the sources of gaseous foam-forming compounds.
The desirability of using carbon dioxide as an auxiliary blowing agent to replace some or all of the water and/or conventional halogenated blowing agents currently in use is apparent, since such additions would not produce urea structures or heat and would not consume expensive isocyanate. Moreover, CO.sub.2 also would not pose any problems of flammability and would decrease the level of toxic or possibly harmful effluents in the plant and atmosphere.
However, the simple addition of carbon dioxide in gaseous form as an added ingredient to the foam formulation at the mixing head is not effective. No measurable effect on either density or firmness is achieved. Several other methods have been proposed for introducing CO.sub.2 into the foam-forming reactants.
For example, it has been suggested in U.S. Pat. No. 3,184,419 that liquified CO.sub.2 be incorporated in a prepolymer mixture in an amount up to four percent by weight. The CO.sub.2 is introduced into the prepolymer stream at 150 atmospheres pressure through an atomizing nozzle. The prepolymer containing CO.sub.2 is fed to the mixer which is maintained at 300 psig. The use of high pressure in conventional mixing chambers is not desirable because of engineering problems. Moreover, there is no disclosure of how the CO.sub.2 is maintained in the mixture once the pressure is reduced as the material is discharged from the mixer to atmospheric pressure. Also there is no indication of the effect of adding liquified low temperature CO.sub.2 to the foam-forming mass. No data on the physical properties of the foam are provided by which the effect of the method on the density of the foam produced can be determined.
In U.S. Pat. No 4,284,728, addition of carbon dioxide is proposed as a stabilizer, i.e., to reduce the reactivity of reactive amines used as cross-linking agents in the foam forming reaction to produce high resilient, or HR, foams. A blend of polyol and diamine cross-linking agent is treated with CO.sub.2, as by sparging, up to a concentration of 2.0 moles of CO.sub.2 per equivalent of cross-linking agent. However, HR foams obtained using the CO.sub.2 treatment exhibited a higher density, rather than a lower density, than those made without CO.sub.2 treatment under comparable conditions.
An apparatus for use in the continuous manufacture of molded parts from flexible polyurethane foam which incorporates a finely divided stable gas dispersion in one of the foam forming components has been disclosed in U.S. Pat. No. 4,526,907. Among the inert gases suggested is CO.sub.2. While the production of a stream containing macroscopic bubbles may be suitable for production of molded parts, the presence of preformed bubbles in the liquid foam-forming composition in the casting of flexible free rise foam is undesirable since it leads to unacceptable defects in the finished product.
It has also been suggested in EPO 145,250 that adducts of CO.sub.2 can be prepared for use as additional blowing agents in polyurethane foam manufacture. The CO.sub.2 adduct is produced by dissolving CO.sub.2 in the polyol, either polyether or polyester, in the presence of at least one low molecular weight fluid, such as water, and certain amines and halocarbon blowing agents of the type customarily used in polyurethane foam production. These adducts are destabilized by reaction with the TDI thereby releasing the CO.sub.2 as a gas in the foam-forming reaction mixture. The method disclosed was limited to the mixing and dissolution of solid and gaseous CO.sub.2 in a pressurized vessel in a polyol that also contained water, trichlorofluoromethane, silicone, tin catalyst and an amine accelerator. While this method appears to permit the reduction of water and isocyanate in the foam-forming reaction, the quality of the final foam products are not fully disclosed. In one example, foam shrinkage was reported. Moreover, the method of incorporating the CO.sub.2 by dissolution in the polyol component which also contains all of the other "low molecular weight fluids" which comprise the formulation can be a severe handicap under the actual operating conditions of the continuous manufacture of flexible foams where it is often necessary to make formulation adjustments on the fly, i.e., by changing the proportion of various component feedstreams entering the mix head.
The use of a CO.sub.2 adduct is also disclosed in U.S. Pat. No. 4,735,970 in the preparation of rigid foams by the frothing process. The adduct is a reaction of CO.sub.2 with specified amines containing at least one secondary amino group, no primary amino groups, and at least one primary or secondary hydroxyl group provided there are not more primary hydroxyl groups than amino groups, and optionally water. Although it is stated that the method can be employed in the manufacture of flexible and semirigid foams by block foaming and the laminator process, no examples are provided other than frothing of rigid foams.
It is therefore an object of this invention to provide methods for use in lowering the density of polyurethane foam products without the use of additional water-isocyanate reactions or auxiliary blowing agents which may be toxic or detrimental to the environment.
It is also an object of the invention to provide methods for reducing the density of polyurethane foams which will be (1) relatively simple and economical to practice; (2) readily adapted to meet changing conditions and formulations as required in the continuous commercial production of such foams; (3) adapted for use with variety of foams including rigid, semi-rigid and flexible, polyether and polyester polyols and polymer polyols, high resiliency foams, and to formulations that incorporate the usual additives, such as colors, plasticizer, fillers, combustion modifiers, and the like. Preferably, such methods should be readily adaptable to existing foam producing equipment.
It is another object of this invention to provide methods for reducing the density of such foams that also exhibit improved compression set and cell structure, that are free of holes caused by other dissolved gases.
It is a further object of the invention to provide a method of producing polyurethane foam of lower density having an improved curing pattern by eliminating or reducing the excessive cooling which accompanies the use of conventional auxiliary blowing agents.
The above objects, and others, have been met by the invention which is described below.